It's not about the head coaches at all in my opinion. It’s just that United at some point stopped progressing (not only as a team of players, but in general) and was simply overtaken by many competitors. In short, United are behind the times. To stay at the top you need to “run” faster than everyone else, but United were not ready for this, Ronaldo described this in sufficient detail in his interview with Morgan.
But this does not change the fact that Ten Hag does not correspond to the level of top clubs and should be sent back to Ajax (along with his "miracle" acquisitions)

If Erik Ten Hag gets to qualify Manchester United fir the Champions League Competition and wins the FA Cup Competition, then all of these flaggings would definitely come to an end.
Lol, I believe you must be amongst those who thinks Gareth Southgate would also be a good idea of replacement for Erik Ten Hag. I won't be fazed.
Manchester United really needs a coach that would change the clubs game play and it actually not Gareth Southgate because Manchester united have got some professional players so all they need now is a coach with good coaching skills and not the one to test his skills. No matter any achievement that Eric Ten Haag will bring to Manchester United, it doesn't mean he can handle the club very well in the future so after this season, the management of Manchester united needs to sort for his replacement and bring a more professional coach that can handle the club effectively.
Had it been Manchester United has a coach that understands the nature of play of the team, they would have gone far earlier because if you watch Manchester United play you will see that they are always more active at the beginning of a game and doesn't seems to maintain same energy till full time so they need a coach that can change that mentality of United's players on the need to maintain same energy they started a game with till the end.